The present invention relates in general to semiconductor manufacturing and, more particularly, to the gettering of impurities in a semiconductor.
In silicon, metal impurities, particularly the heavy metallic impurities such as iron, copper, and nickel, can produce deep energy level sites causing undesirable generation and recombination of minority carriers in the active regions of the substrate. Some metal impurities diffuse very rapidly in silicon at modest temperatures, and due to their low solubility can form precipitates. When these precipitates form at the silicon/silicon dioxide interface, the barrier height for electron transfer from the silicon to the oxide is reduced and the sites become high current density paths that result in premature oxide failure. If these precipitates form at or near a reverse biased P-N junction of the semiconductor device, leakage current from the P-N junction will increase because of the high minority carrier generation rate. Accordingly, device performance degrades, or the semiconductor device may fail.
The metal impurities can arise from a variety of sources. For example, metal impurities may come from placing a material on the wafer surface during processing, or from the processing equipment itself. Once the metal impurities reside on an exposed silicon region of the wafer, subsequent high temperature processing allows the metal impurities to diffuse throughout the substrate. A variety of techniques have been proposed to reduce the concentration of contaminants including ultra clean processing, cleaning processes and gettering.
Ultra clean processing reduces contamination in the manufacturing process thus lowering the level of contamination that reaches the silicon substrate. Ultra clean processing involves sophisticated ultra clean factories, and the use of ultra pure chemicals and gases. While ultra clean processing has been successful in reducing the level of metallic contamination, it is very costly. In addition it is merely a preventative method and provides no means for trapping impurities that enter the silicon.
Various wet, dry and high temperature (pyrochemical) cleaning processes have been developed. While these methods remove impurities from the substrate, some degree of redeposition exists at the end of each cleaning process, thus limiting their effectiveness.
Substrate gettering techniques and structures have been the mainstay of the semiconductor industry. Although several different techniques have been developed, each involves the production of low energy sites that can trap the mobile impurities. Among the most prevalent techniques are various forms of backside damage, including mechanical abrasion, silicon nitride or polysilicon deposition, and argon implantation. More recently, techniques for producing damage on the front side of substrates, but away from active semiconductor areas, have been developed. The front side techniques tend to be more permanent than the backside processes.
Another common method is oxygen precipitation gettering, sometimes called denuded zone intrinsic gettering. In this method a wafer with a specific range of oxygen concentration is processed through particular cycles of heating and cooling to produce denuded zones near the top and bottom surfaces of the substrate and with a high concentration of oxygen precipitates in-between the denuded zones. The oxygen precipitates serve as gettering sites for the mobile impurities.
There are two basic gettering techniques: damage gettering and oxygen precipitate gettering. Unfortunately, oxygen precipitate gettering is not very effective in gettering some impurities such as iron because iron does not form stable precipitates at the oxygen precipitate sites. Hence, the iron impurities may be re-emitted into the silicon substrate upon subsequent processing. Damage gettering loses effectiveness with subsequent high temperature processing which anneals much of the damage. In addition, neither technique is applicable to gettering in polysilicon layers, separated from the substrate by a dielectric layer.
Accordingly, a need exists for gettering mobile impurities that is applicable to silicon and polysilicon layers where the impurities remain gettered during subsequent processing.